1. Field of the Invention
This invention relates to the precipitation of aluminum hydroxide. More particularly, this invention relates to the control of the form of crystals obtained in the precipitation of aluminum hydroxide using either cosolvents or varying caustic concentration or both whereby crystals may be formed which vary from principally acicular to principally lamellar.
2. Description of the Related Art
The use of various process techniques to control the precipitate, e.g., the phase, obtained from a solution comprising sodium aluminate and an alkali hydroxide is well known. For example, Ashley U.S. Pat. No. 2,657,115 teaches the production of a semi-gelatinous aluminum hydroxide precipitate from an alkali metal aluminate solution formed by digesting an aluminum oxide or hydroxide such as bauxite either with sodium hydroxide or potassium hydroxide. The alkali metal aluminate solution is then reacted with a mineral acid to produce a semi-gelatinous aluminum hydroxide while maintaining the pH between 5.8 and about 8.3, preferably between 6.0 and 7.5.
Michel et al U.S. Pat. No. 3,411,877 describes the formation of hydrargillite crystals by the initial precipitation of an alumina gel formed preferably by a process of continuous precipitation with vigorous agitation using a solution of sodium aluminate and the addition of acids for precipitation in an amount and at a rate to maintain the pH within the range of 8 to 9.
Warthen et al U.S. Pat. No. 3,853,789 discloses a process for preparing macroporous alumina extrudates by first obtaining a hydrous alumina gel precipitated from solution either by combining an alkali metal aluminate with an acid or an acid salt, or by precipitating an acid aluminate salt solution by the addition of an alkaline reagent. The precipitation is said to be conducted at a pH of between 6 and 11 and preferably between 7.5 and 8.5.
The use of alcohols to treat Bayer spent liquor solutions is also known. Bayer spent liquor comprises the liquor which remains after precipitation of aluminum hydroxide from a solution or green liquor formed by the "Bayer Process" which comprises the digestion of bauxite ore using a caustic liquor.
Bush U.S. Pat. No. 4,464,347 and Bush et al U.S. Pat. No. 4,496,524, both assigned to the assignee of this invention, describe a treatment with ethanol of a Bayer spent liquor, i.e., a sodium aluminate liquor from a Bayer digestion process atter initial precipitation of aluminum hydroxide therefrom. In these patents, the spent liquor is purified using the ethanol either to promote a precipitation of sodium oxalate impurity (when the ethanol concentration is 50 wt. % or less of the amount of liquor) or to form a two-layer system. In the latter case, the ethanol and at least a portion of the caustic comprise one layer and the remainder of the spent liquor forms the other layer. By removing some of the caustic from the spent liquor, the alumina to caustic ratio in the remaining spent liquor is increased sufficiently to permit a further precipitation to remove further alumina values from the spent liquor. The remaining spent liquor is then further treated to remove other impurities.
Malito et al U.S. Pat. No. 4,430,310 describes a process for purifying Bayer process liquors by treating the liquors, after removal of substantially all of the alumina content, with an organic solvent of relatively low boiling point (such as methanol, propanol, butanol, or mixtures thereof) which causes the precipitation of inorganic carbonate and/or sulfate salt from the treated liquor.
Halff et al U.S. Pat. No. 3,511,606 describes a process for recovering sodium hydroxide from solutions or mixtures containing aluminate. The sodium hydroxide is separated from an aqueous mixture of sodium hydroxide and aluminate by adding ethanol to the mixture to form a light phase containing the ethanol with sodium hydroxide and a little aluminate with most of the aluminate in a heavy phase together with some sodium hydroxide and a little ethanol. The patentee also says that methanol may be added to the sodium hydroxide solution instead of ethanol, in which case, an aluminum hydroxide is formed which precipitates. The precipitated aluminum hydroxide and methanol are then separated from the solution to form a reuseable solution of sodium hydroxide.
The use of alcohols such as methanol and ethanol to accelerate the recovery of alumina from aluminate liquor is discussed in British Patent Specification No. 1,123,184. The patentees state that the addition of a liquid alcohol to the non-alcoholic liquor is thermodynamically a pronounced irreversible process involving a considerable entropy rise and hence loss of energy. The patentees, therefore, introduce the alcohol into the aluminate solution by passing aluminate liquor down an addition column while passing a vapor mixture containing alcohol and steam through the column countercurrent to the aluminate liquor.